Method of manufacturing carbon fibres

ABSTRACT

In the manufacture of carbon fibre a process is proposed in which a polymeric fibre is firstly heated to a temperature in the range 200* - 300* C in an inert atmosphere for a predetermined time, secondly heated to a temperature in the range 150* - 250* C in an oxygen containing atmosphere for a predetermined time, and subsequently pyrolysed. Using this two-stage preoxidation the exothermic nature of the reaction is reduced.

Unite States atent 1 [11] 3 ,862,334

Turner Jan. 21, 1975 [54] METHOD OF MANUFACTURING CARBON 3,412,062 11/1968 Johnson et a1 23/209.1 X

FIBRES Inventor: William N. Turner, Mickleover,

England Secretary of State for Defense, London, England Filed: June 4, 1969 Appl. No.: 330,534

Assignee:

Foreign Application Priority Data June 4, 1968 Great Britain 264-22/68 US. Cl. 423/447, 264/29 Int. Cl C0lb 31/07 Field of Search 23/2091, 209.2, 209.4;

References Cited UNITED STATES PATENTS 11/1966 Tsunoda 23/2091 Primary ExaminerEdward J. Meros Attorney, Agent, or Firm-Cushman, Darby & Cushman Using this two-stage preoxidation the exothermic nature of the reaction is reduced.

5 Claims, N0 Drawings METHOD OF MANUFACTURING CARBON FIBRES This invention relates to a method of manufacturing carbon fibres.

lt has previously been proposed to manufacture carbon fibres by the pyrolysis of various starting materials particularly polyacrylonitrile and co-polymers thereof. In this process it has been proposed to use a preoxidation step in which the starting fibre is heated in an oxidising atmosphere to a temperature of some 250C, this pre-oxidation step giving the starting material at least the characteristics of a cross-linked material and hence rendering it more resistant to tie-polymerisation so that the subsequent pyrolysis step can be carried out more quickly. One disadvantage of the pre-oxidation treatment as previously proposed lies in its exothermic characteristic which resulted in a considerable output of heat and consequent danger of running away of the reaction and explosion or catastrophic charring of the fibre.

We have invented a method by which the exothermic characteristics of the pre-oxidation may be altered to a far more easily controllable form.

According to the present invention a method of manufacturing carbon fibres comprises the steps of firstly heating a polyacrylonitrile fibre in an inert atmosphere to a temperature in the range 200 to 300C, secondly heating the fibre in an oxygen containing atmosphere to a temperature in the range 150 to 250C and subsequently pyrolysing the fibre.

Preferably said pyrolysis step comprises heating the fibre in an inert atmosphere to a temperature in the range 800 to 1,500C; thus the pyrolysis step may involve heating to l,OOC in an atmosphere of nitrogen.

The pre-oxidation treatment preferably comprises the first heating step in nitrogen and the second heating step in air, while the first heating step may comprise heating for 3 hours at 2l0C, 2 hours at 220C and 2 hours at 250C in nitrogen followed by the second step comprising 7 hours at 220C in air.

The present invention is based on the results of tests we have carried out on the pre-oxidation of, particularly, polyacrylonitrile fibres using a technique known as differential thermal analysis. This technique involves heating the material at a constant rate of temperature rise (6C per minute) up to about 500C and accurately measuring the temperature difference between this sample and an inert reference material. Any difference in temperature between the polyacrylonitrile sample and the reference sample indicates either an exotherm or an endotherm depending upon whether the polyacrylonitrile is at a higher or lower temperature than the reference sample.

Carrying out this technique for polyacrylonitrilewhen heated in air, we found that the reaction showed two exothermic peaks at approximately 250 and 310C respectively. It will be appreciated that in the prior art pre-oxidation both these exotherms will con-' tribute to the final energy production of the reaction and hence to its likelihood of running away.

Further tests we carried out with the polyacrylonitrile heated in an inert atmosphere showed that the lower temperature exotherm remains even in inert conditions. We believe that the lower temperature exotherm is caused by the conversion of the nitrile group in the original polymer-by a cross linking mechanism or alternative-1y by a ring closure along the polymer molecule chain, while the higher temperature reaction which requires the presence of oxygen results in dehydrogenation and oxygen absorption by the fibre. These reactions are both useful in conferring the necessary stability on the fibre to reduce de-polymerisation and enable a fast heating rate to be used on the subsequent pyrolysis.

This hypothesis was tested by treating fibres first in nitrogen then in air, and it was found by differential thermal analysis that as expected, each treatment produced a single exothermic reaction of less intensity than the combined reaction which occurs merely by heating in air.

lt is evident from the above that by heating the fibre first in nitrogen and then in an oxygen containing atmosphere according to the present invention, the two exothermic reactions are separated and thus the stabilisation of the polymer can be achieved with much less danger of running away of the reaction.

In an example of the method according to the invention fibres of a polyacrylonitrile co-polymer known as Courtelle and sold by Messrs Courtaulds Limited were heated in nitrogen for 3 hours at 210C, 2 hours at 220C and 2 hours at 250C. The fibre was subsequently placed in an atmosphere of air and heated at 220C for 7 hours. The resulting fibre 'was then pyrolysed by heating to 1,000C in an inert atmosphere over a time of some 3 hours. The resulting carbon fibre was then tested by standard methods to find the values of Youngs modulus and ultimate tensile strength. These values were then compared with values obtained for material which had been pre-oxidised by heating only in air to a temperature of 220C for some 7 hours then pyrolysed in a nitrogen atmosphere for the same length of time and under the same conditions of temperature as with the fibres pre-treatedaccording to the invention. For the fibres pre-treated according to the invention the mean value of Youngs modulus found was 31 X 10 psi while the average ultimate tensile strength was 219 X 10 psi. These values compared with the values for the fibres not pre-treated according to the invention which gave Youngs modulus of 28 X 10 psi and an ultimate tensile strength of 260 X 10 psi. lt will be appreciated that these differences in values are relatively minor and are within the limits of the normal batch-to-batch variations observed using the prior method.

It will be appreciated from these results that the pretreatment according to the present invention is able to produce fibres of similar strength and modulus to those produced by the prior method while reducing considerably the risk of explosion. In addition we have found that using the two stage pre-treatment of the present invention the temperature of the second step (i.e., heating in an oxygen containing atmosphere) may be effectively carried out at a lower temperature than could be used with the prior art pre-treatment thus allowing further gains in controllability of the pretreatment and possibly a shortening in overall time.

It will be appreciated that although the invention has been described above with reference to a particular heat treatment, it would be possible to vary the conditions of the heat treatment considerably. Thus we believe that any temperature within the range 200 to 300C could be used for the first step of heating in an inert atmosphere while any temperature in the range to 250C is likely to be suitable for a second step of heating in an oxygen containing atmosphere. Of the various oxygen containing atmospheres which would be useful for the second step, we find air to be the most convenient since its ready availability outweighs any other consideration.

Again although described above with reference to a particular co-polymer of polyacrylonitrile it will be appreciated that the present invention is of use in relation to any polymer to which this combined cross linking and oxygen absorption process is applicable. Obviously this includes other co-polymers of polyacrylonitrile and is in fact also applicable to cellulosic materials such as rayon and polyamide materials such as nylon.

The subsequent pyrolysis of the fibre may be carried out to any temperature in the range 800 to 1,500C and may also be followed by or include a graphitisation treatment which may be performed at any temperature from the pyrolysis temperature up to some 3,500C depending upon the final properties required of the fibre.

Fibres produced by the method of the present invention may be used in many applications, however, their widest use is as a reinforcing material in a resin matrix, the whole being used as an engineering material for use in such manufactures as gas turbine engines; bearings,

etc.

I claim:

I. A method of manufacturing a carbon fibre comprising the subsequent steps of firstly heating a polyacrylonitrile fibre in an inert atmosphere to a temperature in the range of 200 to 300C during which time an exothermic reaction takes place, secondly heating the fibre in an oxygen containing atmosphere to a temperature in the range 150 to 250C during which time an additional exothermic reaction takes place, and subsequently pyrolysing the fibre.

2. A method as claimed in claim 1 and in which the pyrolysis of the fibre is effected by heating the fibre in an inert atmosphere to a temperature in the range 800 to I500C.

3. A method as claimed in claim 2 and in which the pyrolysis of the fibre is effected by heating the fibre to a temperature of about l,000C in an atmosphere of nitrogen.

4. A method as claimed in claim 1 in which said first heating step comprises heating the fibre for 3 hours at 210 C, 2 hours at 220 C, and 2 hours at 250 C in an atmosphere of nitrogen, during which time an exothermic reaction takes place.

5. A method as claimed in claim 1 and in which said I second heating step comprises heating the fibre in air at a temperature of 220 C for 7 hours during which 

2. A method as claimed in claim 1 and in which the pyrolysis of the fibre is effected by heating the fibre in an inert atmosphere to a temperature in the range 800* to 1500*C.
 3. A method as claimed in claim 2 and in which the pyrolysis of the fibre is effected by heating the fibre to a temperature of about 1,000*C in an atmosphere of nitrogen.
 4. A method as claimed in claim 1 in which said first heating step comprises heating the fibre for 3 hours at 210* C, 2 hours at 220* C, and 2 hours at 250* C in an atmosphere of nitrogen, during which time an exothermic reaction takes place.
 5. A method as claimed in claim 1 and in which said second heating step comprises heating the fibre in air at a temperature of 220* C for 7 hours during which time an additional exothermic reaction takes place. 